As discussed in U.S. Pat. No. 4,027,398, issued to John T. Fowler, et al. on June 7, 1977 and incorporated herein by reference, there is often a need for a remote reading magnetic compass providing an electrical output signal representing compass heading. A particularly effective digital compass is a subject of U.S. Pat. No. 3,888,016, wherein a compass disk contains a plurality of optically coded tracks which are photoelectrically sensed to provide a digital output signal representative of compass heading. The coded disk is supported for rotation about a normally vertical axis by upper and lower pivot bearings which also maintain optical alignment of the photoelectrically sensed disk with respect to the associated sensor. The compass is mounted within a gimball assembly to maintain a horizontal disk orientation and minimize spurious motion of the disk.
While connections to such a compass may be made by electrical leads, it will be appreciated that the compass is prevented by stops from continuously rotating 360.degree. about the roll axis to prevent breakage of the leads from the outer compass housing to the compass. In the past, slip rings or the like have been utilized to permit 360.degree. rotation about the roll axis, but slip ring embodiments are subject to wear and provide an inordinately high spurious signal content for signals transmitted through the slip ring interface.
As exemplified in U.S. Pat. No. 4,047,168, there have also been attempts to transmit signals through the two single point contacts afforded by conductive jewel gimballing along the roll axis. This type of connection interface, while effective at low data rates, is somewhat unreliable at high data rates. The reason is contact bounce which, while not a factor at low 500 Hz data rates, results in lost data at the 20 KHz data rates associated with the above-mentioned remote reading compass. Thus, the reliability of both slip ring and single point contact connection systems is not optimal for high data rate systems.
360.degree. rotation is important when remote reading compasses are supplied in seismic streamers in which the streamer line twists and rotates about the roll axis as it it towed. If 360.degree. roll freedom cannot be provided, the compasses provide false readings when the vertical axis of yaw cannot be maintained. It should be noted that remote reading compasses using spring-loaded conductive pivots and a single cylindrical housing to permit mounting in streamers are available as Digicourse models 318, 319, 320, and 321. Models 225, 226, and 227 utilize a single cylindrical housing with hard-wired connections.
By way of further background, it is noted that the early Anschultz gyroscopes, as exemplified by U.S. Pat. Nos. 1,589,039 and 3,373,617 were provided with a semiconductive fluid between an inner gyro structure and an outer casing. All of these gyros were spherical in design and while 360.degree. rotation was permitted around the yaw axis, rotation about the roll axis was strictly limited. Electrical power was applied to the outer shell at the north and south poles. The electrical power was transmitted through the semiconductor fluid to a phase splitting circuit which provided three phase power for the gyroscope. At that time, the orientation of the gyroscopic device was readout through the use of electrically conductive stripes on the inner and outer shells through a null circuit. It will be appreciated that losses between electrodes on a spherical surface through the semiconductive fluid are large due to the spherical geometry, especially with small miniature spheres. Devices using these small spheres draw considerable amounts of current due to the closeness of the spacing of the electrodes on the sphere and the associated extremely short conduction paths over the surface of the sphere. Small spheres are important to the miniaturization of digital compasses, but their use has heretofore been limited because of the difficulty of coupling signals into and out of the compass and because using a conductive fluid interface poses the problems of large losses and current drain.
In summary, the requirement for unrestricted 360.degree. rotation of the inner portion of the compass about the roll axis presents unusual difficulties in the transmission of such large amounts of information without the use of hard wiring. Since these compasses may be remotely located and are therefore relatively inaccessible, reliability is of paramount importance so that reliable jitter-free digital outputs of compass heading may be obtained.